The secretion of parathyroid hormone (PTH) by the parathyroid gland is directly regulated by extracellular calcium ions (Ca2+) via the Ca2+-sensing receptor. However, there are reports of magnesium (Mg2+) effect on PTH secretion: acute hypermagnesemia can suppress PTH and it is suggested that the mechanism of suppression is similar to that of hypercalcemia. During platelet apheresis, the infused citrate forms complexes with both Ca2+ and Mg2+ ions and, therefore, decreases the concentration of the bioactive forms ("ionized" Ca and Mg) of both cations in the blood returned to the donor. The resulting hypocalcemia and concomitant hypomagnesemia could influence the PTH response and could be responsible for the citrate toxicity symptoms observed during apheresis. We investigated the time course of changes in the concentrations of PTH, citrate, ionized Mg (iMg) and ionized Ca (iCa), and other electrolytes, which occur during leukapheresis. Laboratory analyses of serum and urin specimens were performed during allogeneic peripheral blood progenitor cell donations (n=244) with and without prophylactic Ca. Marked increases in serum citrate concentration occured that were negatively correlated with a significant decreases in iCa and iMg (by up to 56 percent). The renal excretion of citrate was accompanied by significantly increased renal excretion of Ca and Mg. The serum concentration of both Ca and Mg remained decreased 24 hours after the procedure.Symptoms of citrate toxicity were more frequent in donors that had low serum total Mg concentration at the start of procedure. Prophylactic Ca infusions reduced the symptoms and attenuated the decreases in serum potassium.